Download small animal anesthesia parts i and ii

SMALL ANIMAL ANESTHESIA PARTS I AND II
William W. Muir, III, DVM, PhD, DACVECC, DACVA
“There are no safe anesthetic agents; there are no safe anesthetic procedures;
there are only safe anesthetists.”
Robert Smith
Anticholinergics
Anticholinergics are competitive antagonists of acetylcholine at post ganglionic parasympathetic
muscurinic receptors. Stimulation of muscurinic receptors induces salivation, pupillary constriction,
bronchoconstriction, gastric acid secretion, gastrointestinal motility, and slowing of the heart rate. The two
most commonly used anticholinergics in veterinary medicine are atropine and glycopyrrolate (Table 1).
These two drugs differ in their duration of action, with the effects of glycopyrrolate lasting at least twice as
long as atropine. Furthermore, glycopyrrolate does not cross the intact blood brain barrier or the placenta.
Anticholinergics increase myocardial oxygen consumption by increasing heart rate. and can precipitate
cardiac arrhythmias and decrease the threshold for ventricular fibrillation. For these reasons
anticholinergics should not be used as a routine part of the anesthetic regimen, since many critically ill
patients can not tolerate increases in myocardial oxygen consumption or alterations of cardiac rhythm.
Anticholinergics are indicated for specific situations. For example, animals with high resting vagal tone, or
high vagal tone resulting from narcotic or alpha-2 administration, those with excessive upper airway
secretions, or in those animals in which cardiac output is dependent on maintenance of a normal heart rate.
This includes neonates or individuals with cardiac tamponade.
Benzodiazepines
Diazepam and midazolam are benzodiazepines frequently used in small animals. These two drugs
are similar except that midazolam is water soluble while diazepam is solubilized in 40% propylene glycol.
Intravascular injection of diazepam can be associated with pain at the injection site, furthermore the
propylene glycol diluent prevents rapid absorption following intramuscular injection. The elimination half
life of midazolam is shorter than diazepam. Benzodiazepines act by facilitating the actions of GABA and
glycine, two inhibitory neurotransmitters. They can unmask suppressed behavior and can cause increased
agitation and restlessness, particularly in cats. Debilitated and depressed dogs or cats often respond to a
relatively small IV dose of diazepam with profound CNS depression. Endotracheal intubation can often be
accomplished following their use alone. Benzodiazepines produce minimal cardiac and respiratory
depression and initiate antiarrhythmic effects. They are anticonvulsants and skeletal muscle relaxants, and
when used prior to barbiturates or inhalation anesthetics they decrease the dose of drug necessary to induce
and maintain anesthesia. Both diazepam and midazolam can be combined with opioids to produce
neuroleptanalgesia, with ketamine to produce short-term general anesthesia or with inhalant anesthesia to
improve muscle relaxation. They are also useful when used just prior to anesthetic induction drugs such as
the thiobarbiturates, etomidate or propofol.
Opioid Analgesics and Neuroleptanalgesics
The opioid (narcotic) analgesics can be used as adjuncts to general anesthesia as well as for
postoperative pain control. As adjuncts to general anesthesia, they can be used alone for pre-anesthetic
analgesia and behavior modification while in depressed animals they can be used for anesthetic induction
either alone or combined with other drugs. Combined with sedatives (alpha-2 agonists) or tranquilizers
(acepromazine) they induce profound sedation and analgesia. This is referred to as neuroleptanalgesia.
Opioids act by binding to specific opioid receptors in the central nervous system. The opioid receptors
present within the CNS are termed mu, kappa, sigma, and delta, and there is evidence suggesting their
existence in peripheral afferent nerves. Opioids are usually classified as agonists, agonist-antagonists,
partial agonists, or antagonists depending on their activity at the various receptors. Stimulation of the mu
receptor induces analgesia, respiratory depression, miosis, bradycardia, hypothermia, and euphoria.
Stimulation of the kappa receptor produces sedation, analgesia, and miosis. Stimulation of the sigma
receptor induces mydriasis, tachycardia and excitement. The function of the delta receptor is not well
understood. Morphine, meperidine, oxymorphone, and fentanyl are agonists at the mu and kappa
receptors. Naloxone is an antagonist at all three receptors, and butorphanol, pentazocine, nalbuphine, and
buprenorphine are partial agonists at mu and kappa receptors or antagonists at mu receptors and agonists at
kappa and sigma receptors. The overall effect of opioids on behavior in normal conscious dogs and cats
varies from sedation to euphoria and excitement. Administration to debilitated or depressed patients
usually results in sedation.
In general, opioids are powerful respiratory depressants. Experimentally, respiratory depression is
characterized by a delayed response (altered threshold) and decreased sensitivity to increased in carbon
dioxide concentration. Some opioids such as butorphanol demonstrate little or no respiratory depression
due to their receptor specificity. Panting is a common side effects of opioids and is probably related to
drug effects in the thermoregulatory center in the hypothalamus.
Cardiovascular function is well maintained following opioid administration. Left ventricular
contractility, cardiac output, and systemic blood pressure are minimally changed. Some opioids such as
oxymorphone, fentanyl, and morphine can induce bradycardia. Morphine and meperidine initiate histamine
release with resulting peripheral vasodilatation. Histamine release is dose dependent and low doses induce
only mild vasodilatation.
Other notable opioid effects include decreased urinary output due to stimulation of ADH release.
The gastrointestinal effects are variable among species but in the dog generally include vomiting due to
stimulation of the chemoreceptor trigger zone. Dogs initially defecate and there is an increase in flatulence
following opioid administration. Opioids induce an initial GI hypermotility in the dog with an increase in
non-propulsive rhythmic contractions and an increase in smooth muscle tone. This is followed by a period
of GI stasis and constipation.
Opioid agonists that are the most useful in anesthesia of the critically ill patient include morphine,
oxymorphone, and fentanyl. These three drugs can be used alone or as a part of a neuroleptanalgesic
combination (Table 1).
Hydromorphone is approximately 7 times more potent than morphine. An IV bolus of hydro can
induce anesthesia in depressed dogs, however used alone in healthy alert dogs it does not produce loss of
consciousness. Diazepam can be administered IV just prior to hydromorphone for additional relaxation
and sedation. Alternatively midazolam and hydromorphone can be administered IM for pre-anesthetic
neuroleptanalgesia. Like morphine oxymorphone can induce vomiting.
Fentanyl is approximately 50-100 times more potent than morphine and has a 15-30 minute
duration of action. It minimally depresses cardiovascular function. A small IV bolus (0.005-0.01 mg/kg)
of fentanyl administered every 15-30 minutes during inhalation anesthesia provides additional analgesia
and allows a lesser concentration of the inhalant to be used. Fentanyl induced bradycardia can be treated
with an anticholinergic. Fentanyl is also available in combination with the tranquilizer droperidol. This
neuroleptanalgesic drug combination is useful IM and IV for sedation or induction. Other than the
potential for a fentanyl induced bradycardia, the cardiovascular depressant effects are minimal.
Alpha-2 Agonists
The alpha-2 agonists include xylazine, detomidine, medetomidine and romifidine. These drugs
produce the majority of their pharmacologic effects by stimulating alpha-2 receptors and are used clinically
for sedation, muscle relaxation and analgesia. Larger dosages produce a variety of side effects including
repiratory depression, bradycadia and bradyarrhythmias (first and second degree heart block),
hyperglycemia, decreases gut motility and diuresis. Vomiting is a common side effects following
intramuscular administration to dogs and cats. When administered clinically in small dosages these drugs
are excellent for preanesthetic medication or as adjuncts to general anesthesia. The most important point
being that they be administered in small dosages (approximately 1/10 of the label recommendation). All of
the effects of the alpha-2 agonists can be reversed by administration of an alpha-2 antagonist; yohimbine,
tolazoline, atipamezole.
Injectable Anesthetics
The barbiturates, cyclohexamines (ketamine, Telazol® ), propofol and etomidate are injectable
anesthetics use to produce short-term (min.) anesthesia or for induction to general anesthesia with an
inhalant.
Barbiturates
The ultrashort acting barbiturates thiopental and methohexital are pharmacologically similar. These
drugs are cleared from the body by metabolism, and/or redistribution. The duration of action of thiopental
(thiobarbiturates) are in large part dependent on redistribution from the central nervous system into muscle
and fat. This gives them a predictably short duration of action (5-15 minutes) in normal healthy dogs and
cats (Table 2). Induction of anesthesia with the thiobarbiturates and methohexital is extremely rapid and
struggle free following IV bolus administration providing the patients are properly preanesthetized. The
thiobarbiturates are alkaline in solution (pH=10-11), and accidental perivascular injection can result in
inflammation and necrosis at the injection site. Methohexital differs from the thiobarbiturates in that it is
cleared more rapidly from the plasma. There are also breed differences in pharmacokinetics, with the
thiobarbiturates having an unacceptably long duration of action in sight hound breeds. Because these
drugs are highly lipid soluble weak acids that are highly protein bound, their pharmacokinetic behavior is
altered by changes in acid-base balance, albumin content, and the concurrent administration of other drugs.
Acidosis increases the amount of active non-ionized drug, similarly a decrease in plasma protein binding
induced by hypoalbuminemia or the presence of other drugs highly bound to albumin increases the amount
of active drug. For these reasons the barbiturate dose should be reduced in critically ill patients that are
acidotic or hypoalbuminemic. The thiobarbiturates induce a dose dependent CNS depression, resulting in
a dramatic decrease in cerebral metabolic oxygen consumption, cerebral blood flow, and intracranial
pressure.
Barbiturates cause a dose related depression of respiration. The response to inspired carbon
dioxide is depressed, and the respiratory response to hypoxia is diminished. Apnea is common following
an IV bolus.
The barbiturates depress cardiac function. Cardiovascular depression varies with dose and rate of
administration. Bolus injection to normal dogs results in tachycardia, increased peripheral vascular
resistance, a transient increase in systemic arterial pressure, and increased myocardial oxygen consumption.
Left ventricular contractility decreases. Large dosages of thiobarbiturates can induce ventricular
arrhythmias or bradyarrhythmias. These arrhythmias are transient and normal sinus rhythm usually
returns within 10-15 minutes of administration. The thiobarbiturates also enhance the arrhythmogenic
effects of epinephrine in halothane anesthetized dogs. The incidence of arrhythmias is decreased by
simultaneous administration of lidocaine, or prior administration of acepromazine.
Propofol is a nonbarbiturate ultrashort acting intravenous anesthetic that can be administered as a
bolus or by infusion due to rapid clearance. It is similar to the thiobarbiturates but not as cumulative.
Unlike the barbiturates it does not sensitize the myocardium to catecholamine induced arrhythmias and is
less likely to produce respiratory and cardiovascular depression at clinically recommended dosages.
Etomidate is a nonbarbiturate, non-narcotic, ultrashort acting intravenous anesthetic. Anesthesia is
rapidly induced following IV administration, and its hypnotic duration of action is approximately 5-10
minutes. Etomidate is rapidly metabolized in the liver and plasma by non-specific esterases. This rapid
metabolism makes it suitable for use in patients with liver dysfunction. Etomidate causes minimal changes
in cardiac contractility, heart rate, cardiac output, and systemic blood pressure are minimally effected.
Etomidate produces mild respiratory depression. Side effects from etomidate infusion include pain at the
injection site, myoclonus, and gagging and retching. These effects are lessened by premedication with a
tranquilizer or sedative. Etomidate suppresses adrenocortical function for 2-6 hours following a single
bolus dose and may be contraindicated in debilitated or immunosuppressed patients. Etomidate is useful
for anesthetic induction in patients with cardiovascular instability.
Propofol
Dissociative Anesthetics
The most commonly used dissociative anesthetic in veterinary medicine is ketamine. Ketamine is
used in a variety of species, either alone or in combination with tranquilizers and sedatives. The term
dissociative anesthesia implies a pharmacologic uncoupling of sensory input from conscious sites within
the brain. The mechanism of ketamine’s CNS and analgesic effects is unclear but may be related to
decreases in brain acetylcholine, facilitation of GABA, or stimulation NMDA and opioid receptors.
Ketamine produces waxy limb rigidity, wide eyed stare, maintenance of a corneal and laryngeal reflexes,
and occasional violent recoveries. The drug is unsuitable for use in dogs unless it is combined with a
sedatives or tranquilizers. Most typically ketamine has been combined with xylazine, acepromazine, and
diazepam in dogs and cats.
An advantage of ketamine is its relative lack of cardiovascular depression. Ketamine typically
increases heart rate, cardiac output, cardiac contractility, and systemic blood pressure in dogs and cats.
These effects are due at least in part to a centrally mediated increase in sympathetic tone. Because of the
cardiovascular sparing effect ketamine is useful for anesthesia of the critically ill patient. Ketamine,
however can induce cardiovascular depression or tachycardia in patients with limited cardiovascular
function (heart failure, hemorrhagic shock) and maintenance of cardiovascular function should not be
assumed when ketamine is administered to compromised patients.
Ketamine produces does dependent respiratory depression characterized by decreased tidal volume,
increased frequency, and apneustic breathing. The depression is transient and of no consequence in
normal healthy patients, however patients with pre-existing cardiopulmonary depression may experience
severe respiratory insufficiency following ketamine. Ketamine is a potent bronchodilator, and has been
used for anesthesia in asthma patients
The combination of ketamine and diazepam or midazolam is useful for short-term chemical restraint
or anesthetic induction in critically ill patients. The combination is associated with minimal
cardiopulmonary depression, and is administered as a 1:1 mixture by volume, 1 ml of the combination per
9 kg body weight. Analgesics (butorphanol, oxymorphone, alpha-2 agonists) should be added to this
combination to enhance analgesia.
Etomidate
Inhalation Drugs
Inhalation anesthetics are the cornerstones of long-term (greater than 15 minutes) anesthesia of the
critically ill patient. Their duration of action is not appreciably dependent on metabolism and anesthetic
depth can be rapidly adjusted compared to injectable anesthesia. Extremely debilitated patients can be
rapidly and safely “masked down” with relatively low concentrations of the inhalants. Patients that are
alert should be induced to anesthesia using an injectable technique and then receive an inhalant for
anesthetic maintenance. The inhalation drugs in common use include nitrous oxide and the volatile
anesthetics methoxyflurane, halothane, isoflurane and sevoflurane. Enflurane is rarely uses in veterinary
practice and will not be discussed.
Nitrous oxide is a gas that can not be used by itself to sustain anesthesia. It is used concurrently
with the volatile inhalation anesthetics to provide analgesia and sedation with relatively minor
cardiopulmonary depression. Nitrous oxide is very insoluble in blood and other tissues which is
responsible for its extremely rapid onset and termination of action. Nitrous oxide hastens uptake of
concurrently used volatile anesthetics (second gas effect). This is advantageous during induction with a
soluble drug like methoxyflurane. This advantage is minimal with the relatively insoluble halothane or
isoflurane. Nitrous oxide is approximately 30 times more soluble in the body than nitrogen. Since
nitrogen is a major component of gas spaces within the body, nitrous oxide moves into these gas filled
spaces faster than nitrogen moves out. This results in an increase in volume or pressure of the space. For
this reason nitrous oxide is contraindicated for use in patients with pneumothorax or bowel obstruction.
Nitrous oxide leaves the body rapidly following termination of anesthesia. If the patient is hypoventilating
and breathing room air, hypoxia may ensue. For this reason the patient should be maintained on 100%
oxygen in the immediate post operative period if nitrous oxide was a component of the anesthetic regimen.
Halothane, isoflurane and sevoflurane are similar in their effects on the different body systems.
They all produce a dose dependent cardiovascular depression. However, at equally potent analgesic
concentrations isoflurane and sevoflurane produce the least depression followed by halothane,
methoxyflurane. Isoflurane induces peripheral vasodilatation and cardiac output is maintained by an
increase in heart rate. A disadvantage of halothane is that it sensitizes the heart to the arrhythmogenic
effects of catecholamines. Because of this halothane is contraindicated for use in patients prone to
development of cardiac arrhythmias.
Halothane, isoflurane and sevoflurane produce dose related ventilatory depression, with halothane
producing the least depression. Respiratory depression is associated with an increased arterial carbon
dioxide concentration and respiratory acidosis. For this reason, patients with a metabolic acidosis,
concurrent respiratory dysfunction, increased intracranial pressure, or any other condition in which
respiratory depression in undesirable should be ventilated during inhalation anesthesia.
Table 1. Anesthetic drugs commonly used for premedication in critically ill patients
Drug
Dosage (mg/kg)
Comments
Anticholinergics
Atropine
0.02-0.04 IM, SC, IV
Glycopyrrolate
0.01 IM, SC, IV
Benzodiazepines
Diazepam
0.2-0.3 IV
Midazolam
0.2-0.3 IM, IV
Opioids
Hydromorphone
0.05-0.1 IM
Bradycardia should be anticipated
Butorphanol
0.2-0.4 IM
Morphine
0.2-0.4 IM
Bradycardia should be anticipated
Alpha-2 Agonists
Xylazine
0.1-0.2 IV
Bradycardia should be anticipated
Medetomidine
2-10 µg IV
Bradycardia should be anticipated
Neuroleptanalgesics
Midazolam and
0.03 IM
Can combine in same syringe
Oxymorphone
0.05 IM
Bradycardia should be anticipated.
Midazolam and
0.3 IM
Can combine in same syringe
Butorphanol
0.2 IM
Dissociatives
Ketamine
2.0-4.0 IM
Cats only
Table 2. Parenteral anesthetic drugs
Drug
Dosage (mg/kg, IV)
Comments
Barbiturates
Thiopental
8-15
Lidocaine/Thiopental
4 mg/kg of each in succession Do not mix in same syringe
Methohexital
3-5
Should be preceded by a sedative,
tranquilizer or narcotic
Dissociative Combinations
Ketamine/diazepam
1ml/9 kg of 1:1
Ketamine/midazolam
(volume/volume) mixture
Ketamine
2-6
Cats only
Opioids and Neuroleptanalgesic
Hydromorphone
0.05-0.1
Will induce anesthesia only in very
sedate or depressed patients
Fentanyl
0.002-0.004
Used intraoperatively to supplement
inhalation anesthesia
Midazolam/Hydromorphone 0.05 Midazolam
0.05-0.2 oxymorphone
Others
Etomidate
1.0-2.0
Precede with tranquilizer, sedative
or narcotic
Propofol
200-400 µg/kg/min
Similar to thiobarbiturates
Ketamine/Diazepam
2.5/0.1
Poor muscle relaxation
Table 3 Volatile Inhalation Anesthetics *
Approximate Vaporizer Setting
Drug
Induction (%)
Maintenance (%)
Halothane
1-3
0.5-1.5
Isoflurane
1-3
0.5-2.0
Sevoflurane
3-5
1.5-2.5
*Assumes fresh gas flow rate of 20-40 ml/kg/min oxygen. If nitrous oxide is added to the fresh gas, flow
rate then decrease vaporizer setting by approximately 0.25-0.5.
These settings are for a precision vaporizer located outside the anesthetic circle. Vaporizer settings for inthe-circle, on precision vaporizers are not listed.